In recent years, production of circuit wirings of electronic devices has required a technology that realizes high-density packaging (formation of fine circuits) at low costs in order for the electronic devices to fulfill the need of ubiquitous age. An example of such a technology known in the art is to form a conductive fine pattern wiring by screen-printing a silver paste containing nanometer-size silver particles (hereinafter, “nanosilver”) followed by baking the applied paste at a low temperature of 150° C. or less.
A printing technique reduces the number of process steps and achieves a high throughput and thereby can offer circuit wirings at low costs. If low-temperature baking at 150° C. or less is possible, a commodity plastic such as polyethylene terephthalate (PET) or polyethylene naphthalate (PEN) that has low heat resistance but costs less, can be easily formed into thin films, and can be easily made flexible can be used as a substrate material instead of conventional expensive polyimides. Furthermore, use of a silver paste containing nanosilver enables high-resolution patterning and contributes to realizing high-density packaging. This is because whereas a conventional silver paste containing micrometer-order silver may clog a fine-pitched screen mesh needed for high-resolution printing, a silver paste containing nanosilver has no such drawback.
Under such a circumstance, various types of conductive pastes for printing have been proposed. Various types of conductive pastes for screen printing that contain metal nanoparticles protected with a polymer compound containing a basic nitrogen atom, a deprotecting agent for the metal nanoparticles, metal particles larger than the metal nanoparticles, and an organic solvent have been known as the conductive paste for screen printing.
For example, PTL 1 discloses a silver paste that contains nanosilver particles having an average particle diameter of 0.1 μm or less and silver particles having an average particle diameter of 1 to 20 μm as a silver component. PTL 2 discloses a silver paste that can be baked at a low temperature and contains nanosilver particles having an average particle diameter of 0.001 to 0.1 μm and silver particles having an average particle diameter of 0.01 to 0.5 μm as a silver component. PTL 3 discloses a silver paste that can be baked at a low temperature and contains nanosilver particles having an average particle diameter of 1 to 100 nm and silver particles having an average particle diameter of 0.1 to 10 μm as a silver component.
However, the silver paste disclosed in PTL 1 must be baked at a temperature of 200° C. or more to obtain a volume resistivity of 10−5 Ωcm and thus is difficult to print on a plastic substrate having insufficient heat resistance. The silver pastes disclosed in PTL 2 and 3 can be baked at a lower temperature than in the related art and can be printed on a plastic substrate having insufficient heat resistance; however, the volume resistivity after low-temperature baking is somewhat high and is unsatisfactory.
In other words, a conductive paste with which a circuit wiring having a lower volume resistivity can be formed on a plastic substrate having a lower heat resistance by low-temperature baking has not been known.